Divalent europium activated alkaline earth aluminum fluoride luminescent materials and process

ABSTRACT

SRALF5, BAALF5, AND BA3AL2F12, ALL ACTIVATED WITH DIVALENT EQUROPIUM SUBSTITUTING FOR THE ALKALINE EARTH METAL, ARE SHOWN TO BE EFFICIENT PRODUCERS OF SHARP LINE ULTRAVIOLETE EMISSION RESULTING FROM 4F TO 4F ENERGY LEVEL TRANSISTIONS IN THE DIVALENT EUROPIUM, AS CONTRASTED TO PREVIOUS LUMINESCENT MATERIALS UTLIZING DIVALENT EUROPIUM AS AN ACTIVATOR IN WHICH THE EMISSION IS BAND DUE TO 5D TO 4F TRANSITIONS. SOLID STATE AND PRECIPITATION METHODS FOR PRODUCING THESE PHOSPHORS ARE DISCLOSED, THESE MATERIALS ARE USEFUL A PHOSPHORS IN CATHODE RAY TUBES, LAMPS, AND OTHER APPLICATIONS.

,QfL/iT/Vf INTENSITY RELATIVE INTENSITY Dec. 28, 1971 M V. HOFFMAN DIVALENT EUROPIUM ACTIVATED ALKALINE EARTH ALUMINUM FLUORIDE LUMINESCENT MATERIALS AND PROCESS Filed April 6, 1970 Fig 1. F1 5% Z.

EMISSO/V 5P5 C TPUM fM/55/04V SP5 C TPUM 25375 EXC/TFIT/ON 25375 zxc/mr/o/v 85200 541 51-770 =EU I u E k. E i .1 3 4 l l I I l I 335 355 375 355 335 355 375 395 WAVELENGTH, 77.777. WAVEL ENGTH, mm.

Fig 3. w

EMISSION SPECTRUM 4 25379 EXC/TFlT/ON Bag/Hz 62 Eu 2 EMISSOION SPECTRUM f Z5379 EXC/THT/ON 53 Sr 045 :55 1 5 k E g m m J l I J 355 375 335 4/5 71/5 VfL fNG TH, 77. m.

b9 HeT-A torneg United States Patent 3,630,945 DIVALENT EUROPIUM ACTIVATED ALKALINE EARTH ALUMINUM FLUORIDE LUMINESCENT MATERIALS AND PROCESS Mary V. Hoffman, South Euclid, Ohio, assiguor to General Electric Company Filed Apr. 6, 1970, Ser. No. 25,760 Int. Cl. C09k N04 US. Cl. 25230l.4 R 10 Claims ABSTRACT OF THE DISCLOSURE SrAlF BaAlF and Ba Al F all activated with divalent europium substituting for the alkaline earth metal, are shown to be eflicient producers of sharp line ultraviolet emission resulting from 4 to 4 energy level transitions in the divalent europium, as contrasted to previous luminescent materials utilizing divalent europium as an activator in which the emission is a band due to 5d to 4 transitions. Solid state and precipitation methods for producing these phosphors are disclosed. These materials are useful as phosphors in cathode ray tubes, lamps, and other applications.

BACKGROUND OF THE INVENTION This invention relates to line emitting luminescent materials which produce ultraviolet radiation. More particularly, it relates to such materials which are specific alkaline earth aluminum fluorides activated with divalent europium.

Divalent europium as an activator in phosphors generally produces broad band emission through the blue and green portions of the visible spectrum.

Several alkaline earth aluminum fluoride compounds which are not activated to luminescence are known in the prior art. Also, some such fluorides are known to be capable of producing luminescence when activated with certain activators.

Sharp line emitters in the near ultraviolet region can be useful for certain purposes such as in photo-copying devices and possibly in lasers, and they can also be used for the same purposes as band emitters giving radiation in the same regions. There are some ultraviolet emitting materials available on the market, but they are not as efiicient as would be desirable, and ultraviolet line emitters are not generally available. Also, the literature does not suggest suitable means for providing such more efiicient materials.

SUMMARY OF THE INVENTION It is an object of the present invention to provide efl"1- cient line emitting luminescent materials which emit in the near ultraviolet region. A further object is to provide processes for producing these materials.

Briefly stated, the present invention provides alkaline earth aluminum fluoride luminescent material activated to luminescence by divalent europium and selected from group consisting of the following: M Eu AlF wherein: M=Sr, Ba, Sr-l-Ba, or Sr Ca wherein y 0.5; x is from a small but effective amount to produce luminescence up to 0.30; and M' Eu Al F wherein: M is Ba +Sr in which y is from zero up to about 0.1; and z is from a small but eifective amount to activate to luminescence up to 0.30.

The preferred concentrations of the divalent europium activator are in the range of 0.02 to 0.08 in each of the activated compounds of the invention. Percentages and proportions herein are given in molar quantities, except where indicated otherwise. The permissible variations in composition by substituting different alkaline earth elements as indicated above do not cause material changes 3,630,945 Patented Dec. 28, 1971 BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a graph of the relative intensity over a range of wavelengths of BaAlF zEu using 2537 angstrom units (A.) wavelength excitation. The spectral band width of 10 A. shows the general structure of the spectra, although it does not resolve out each of the individual lines which give rise to the peak, or the emission which make up its base structure. The notation in which the formula for a compound is given first, followed by a semicolon and the identification of the activator, indicates generally and in particular throughout this specification that the activator is incorporated into the compound in substitution for certain of its elements. In the compounds of the present invention, the divalent europium substitutes for proportional amounts of the alkaline earth metal.

FIG. 2 is a similar emission spectrum for SrAlF :Eu

FIG. 3 is a similar emission spectrum for Ba Al F :Eu

FIG. 4 is a similar emission spectrum for Ba Sr AlF :Eu

DESCRIPTION OF THE PREFERRED EMBODIMENTS In phosphors of the invention according to the formula MalF zEu where M can be Sr or Ba, SrAlF and BaAlF have different structures, but both have an emission in which the 4f4f transitions predominate. When M is a combination of Sr and Ba, with up to about Ba in the Sr compound the result is a solid solution of Ba in the tSrAlF; structure, which changes the size of the unit cell of the compound while retaining the same structure. Very small amounts such as a few percent of Sr can be accommodated in the Ba compound and do not change the structure. In the Sr Ba, AlF :Eu phosphor with x up to 0.85, the addition of Ba results in an emission consisting of both the 4f to 4 line emissions and the 5d to 4 band emissions of divalent europium. In the MalF compounds, when M is Sr, up to one-half of the Sr can be substituted by Ca, with a similar change occurring in the structure and in the emission.

In the M Al F zEu compound, M is primarily Ba with some permissible substitution of Sr for Ba without material change in the luminescent characteristics of the resulting product.

The emission spectra of the three principal phosphors of the invention are shown in FIGS. 1-3, and the emission spectrum of a mixed phosphor is shown in FIG. 4, as discussed above. Resolved spectra show large numbers of individual lines making up both the peaks and part of the bases of the peaks in the spectra. Parts of the bases also consist of some emission from 5a to 4 band transitions. In the SrAlF :Eu phosphor, the spectrum as shown in FIG. 2 consists of 4] to 4] lines superimposed on the 5d to 4 band emission. In the phosphors BaAlF zEu and Ba Al F :Eu, most of the emission is from the 4 to 4 lines and little or none can be detected from 5d to 4] band transitions. The spectra in FIGS. 1 and 3 show less emission in the base structure and a greater amount of emission in the line, when comapred with FIG. 2.

The type of spectra obtained from the phosphors MAlF where M is a mixture of Sr-l-Ba or Sr+Ca is illustrated in FIG. 4. In these phosphors, the addition of a difierent alkaline earth cation changes the energy level transitions to introduce more emission from the a to 4; levels, and further, this emission is at different wavelengths from the 5a to 4 emission found in the SrAlF :Eu phosphor. This change in emission with slight changes in composition illustrates the sensitivity of the 4f-4f transitions of Eu to the structural position in which the Eu is found. Generally, these phosphors absorb exciting energy at least to some extent in the 5d band and then, if the 5d band is enough above the energy levels of the 4] band, the energy is transmitted to the 4f band. Transitions within the 4 band apparently produce the line emission which is characteristic of the materials of the present invention.

This emission behavior in terms of energy level transfers is very unusual and scientifically interesting as well as of potential commercial value in various fields.

Two types of methods have been devised for preparing the luminescent materials of the invention. Examples given hereinafter show the reparation of these materials with the Eu activator level at 0.02 mole. Both the solid state and the precipitation methods can be used to effectively produce the materials when the alkaline earth metal used is Sr or Ba or both together. When Ca is also present in the MAlF compounds, only the solid state method is preferred.

In the solid state method, the quantities of ingredients indicated below in Table I can be used to prepare each of the phosphors shown.

TABLE I.SOLID STATE METHOD INGREDIENTS BaF and SrF are available commercially. AlF is prepared by firing Al O in anhydrous HP at temperatures in the range of 700-800 C. for several hours. EuF is prepared from Eu O by similar firings in a temperature range of 9001000 C. The preferred source of EuF BaF and SrF is first prepared as a coprecipitate of the carbonate or oxalate, combining Eu with Sr or Ba in the proportions required. This carbonate or oxalate is fired in air to the oxide and then retired in anhydrous HF at 800- 900 C. to produce a mixture of the BaF or SrF with the EuF The alkaline earth europium fluoride s then mixed with the aluminum fluoride, either dry or in acetone. The mixed materials are then first fired in a gas consisting of nitrogen with about 1% hydrogen at about 800 C. for several hours such as 4 hours to form the desired compounds and to incorporate europium in the divalent state. The divalent alkaline earth metals in these compounds facilitate the incorporation of the europium in the divalent state, and the trivalent aluminum minimizes the likelihood of the europium being present in the trivalent state. The material is then fired a second time, this time in anhydrous HP at 700800 C. for several hours such as 750 C. for 2 hours. The HP atmosphere used in all instances is preferably 30-50% HF in N by volume. As is known in the art, platinum boats, hardware and atmosphere chambers are preferably used for HF firing of these materials.

In addition to the solid state preparation described above, these materials may alternately be prepared by a precipitation procedure using the ingredients indicated in Table II below in gram quantities, sufficient to produce the molar amounts equivalent to those shown in Table I.

TABLE II.-PRECIPITATION METHOD INGREDIENTS All of these materials are commercially available.

The Eu O is dissolved in dilute HNO All the other ingredients are dissolved in about 400 milliliters (ml.) H O for each material, and the alkaline earth metal, Al and En nitrate solutions are combined by pouring them together. The solution is heated to 7090 C. and the pH is adjusted to between 6 and 7 by adding NH OH as required. The fluoride solution is then added to the nitrate solution, forming a precipitate. This is heated for a sufficient time to digest the precipitate and produce larger crystals that can be filtered more effectively. From 20 minutes to several hours at 7090 C. is effective for digestion, depending on the particle size sought. Then it is filtered, washed with water and dried. The same firing procedure is used as was described above for the solid state procedure.

As will be apparent to those skilled in the art, for the precipitation procedure, subsequent difficulties from excess fluoride can be avoided by using a slight deficiency of fluoride in the ingredients to assure stoichiometric products without excess fluorides. For BaAlF :Eu and srAlF zEu, a convenient amount of fluorides for the precipitation ingredients is slightly less than the theoretical such as about -95% of the amounts indicated in the above descriptions which are calculated to provide essentially stoichiometric materials. For Ba Al F :Eu, the fluoride quantity is equal to or more than theoretical, such as -110% of stoichiometric, to assure formation of the desired compounds.

Since AlF cannot be precipitated effectively or with good control, it was a surprise to find that the precipitation method of the present invention was successful in producing such good material. Apparently the precipitation produces some type of hydrated alkaline earth aluminum fluoride which converts to the desired materials of the invention on subsequent firing of the mixed fluorides precipitated together.

Although nitrate solutions are quite satisfactory for the precipitation process of the invention, other types of solutions can also be used. Chloride solutions of Sr and Ca constituents are suitable, although they would not be as desirable for producing Ba fluoride materials because of the formation of barium chloro-fiuoride compounds.

The foregoing is a description of illustrative embodiments of the invention, and it is applicants intention in the appended claims to cover all forms which fall within the scope of the invention.

The emissions of these materials have been measured in applications such as fluorescent lamps and also under cathode ray excitation. When compared with standard ultraviolet emitting phosphors such as Pb-activated barium silicates (blacklight phosphor), the energy output in the ultraviolet is generally higher for the line emitting Eu activated fluorides of the invention with cathode ray of 2537 A. excitation.

What I claim as new and desired to secure by Letters Patent of the United States is:

1. Alkaline earth aluminum fluoride luminescent material activated to luminescence by divalent europium and selected from the group consisting of the following:

(a) M Eu AlF wherein M=Sr, Ba, Sr+Ba, or Sr Ca wherein y 60.5, and x is from a small but effective amount to produce luminescence up to 0.30, and

(b) M Eu Al F wherein M is Ba "+Sr in which y is from zero up to about 0.1, and z is from a small but effective amount to activate to luminesence up to 0.30.

2. A luminescent material according to claim 1 having essentially the formula: Sr Eu AlF wherein x is from a small but effective amount to produce luminescence up to 0.30.

3. A luminescent material according to claim 1 having essentially the formula: Ba Eu AlF wherein x is from 5 a small but efiective amount to produce luminescence up to 0.30.

4. A luminescent material according to claim 1 having essentially the formula: Ba Eu Al F wherein z is from a small but elfective amount to activate to luminescence up to 0.30.

5. A luminescent material according to claim 2 having essentially the formula: Sr Eu AlF wherein X is between 0.02 and 0.08.

6. A luminescent material according to claim 3 having essentially the formula: Ba Eu AlF wherein x is between 0.02 and 0.08.

7. A luminescent material according to claim 4 having essentially the formula: Ba Eu Al F wherein z is between 0.02 and 0.08.

8. A process for producing the material of claim 1 wherein a solution of the desired proportions of alkaline earth metal, aluminum, and europium constituents is prepared, and a soluble fluoride is added to said solution to precipitate mixed fluorides of said constituents, and said mixed fluorides are digested to increase their particle size and then dried and fired to produce the desired materials.

9. A process according to claim 8 wheerin said soluble fluoride is NH F-HF.

10. A process according to claim 8 wherein said solution of constituents is a nitrate solution.

References Cited UNITED STATES PATENTS 2,372,071 3/1945 Fernberger 25230l.4 S 2,450,548 10/1948 Gisolf et a1 25230l.4 R 2,746,933 5/1956 Smith 252-301.4 R 3,448,056 6/1969 Chenot 252301.4 R

ROBERT D. EDMONDS, Primary Examiner 

